1. Field of the Invention
This invention relates generally to antennas, and more particularly to antennas that are adapted for use with electronic article surveillance systems or other similar utilization devices. In such systems, the articles being protected are tagged with a tag containing a resonant circuit or other electronically detectable device. Typically, a swept frequency interrogation transmitter whose frequency is swept through the resonant frequency of the tag includes a transmitting antenna located near an exit from a protected area. A receiving antenna is disposed near the transmitting antenna and forms a passageway with the transmitting antenna through which someone exiting the protected area must pass. The receiving antenna is coupled to a receiver that detects the signal radiated by the tag whenever the transmitter frequency passes through the resonant frequency of the tag.
2. Description of the Prior Art
Various antennas usable for electronic article surveillance purposes are known. One such antenna is disclosed in U.S. patent application Ser. No. 07/092,052, filed on Aug. 31, 1987 by John R. Feltz et al., now U.S. Pat. No. 4,872,018. Other such antennas are disclosed in U.S. Pat. Nos. 4,251,808 and 4,751,516 to Lichtblau. The above-described application and patents disclose twisted loop antennas designed for electronic article surveillance purposes. The antenna disclosed in the Feltz et al. patent application is fabricated from two twisted loops of coaxial cable that are interleaved to form a multiple loop antenna, while the antennas disclosed in the Lichtblau patents are fabricated in the form of a multiple twisted loop configuration contained within a metal tube. The metal tube of Lichtblau and the shield of the coaxial cable of Feltz et al. , now U.S. Pat. No. 4,872,018, act as electric field shields.
While the antennas disclosed in the Feltz et al. application and in the Lichtblau patents do provide a way to detect a tag passing through an exit from a protected area, both systems have difficulty in detecting a tag when it is passed by the antennas in certain orientations, and both systems have been found to be responsive to certain spurious signals generated by extraneous signals other than tags.